1. Field of the Invention
The present invention relates to a catalyst for use in catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase. More particularly, the present invention is concerned with an oxide catalyst for oxidation or ammoxidation, which comprises an oxide containing, in specific atomic ratios, molybdenum (Mo), vanadium (V), niobium (Nb) and antimony (Sb), wherein the oxide catalyst has a reduction ratio of from 8 to 12% and a specific surface area of from 5 to 30 m2/g. The present invention is also concerned with a process for efficiently producing this catalyst. The catalyst of the present invention is advantageous not only in that the selectivity for and yield of the desired product in the oxidation or ammoxidation are high, but also in that the catalyst exhibits only a small lowering of the yield of the desired product even in a long reaction time. Therefore, when the catalyst of the present invention is used for performing a catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase, an unsaturated carboxylic acid or an unsaturated nitrile (namely, (meth)acrylic acid or (meth)acrylonitrile) can be produced stably in high yield for a long period of time. Further, since the catalyst of the present invention exhibits only a small lowering of the yield with the passage of reaction time, the catalyst of the present invention is also advantageous in that, when a molybdenum compound is added to the catalytic oxidation or ammoxidation reaction system as conventionally practiced in the art for the purpose of maintaining a high yield by preventing a catalyst degradation caused by the volatilization or escaping of molybdenum from the catalyst, the amount of molybdenum compound added and the frequency of addition of molybdenum compound can be decreased, as compared to the case of the use of conventional catalysts, so that the reaction can be performed economically. In addition, the catalyst of the present invention is advantageous in that a moderate catalyst activity can be exhibited, and hence there can be prevented problems that too large an amount of catalyst is required for the reaction, thus causing too heavy a load on the reactor and that the heat of reaction generated becomes too large, rendering it impossible to effect a satisfactory heat removal from the reaction system.
2. Prior Art
Conventionally, there have been well known a process for producing (meth)acrylonitrile by ammoxidation of propylene or isobutylene, and a process for producing (meth)acrylic acid by oxidation of propylene or isobutylene. Recently, as substitutes for such processes for the oxidation and ammoxidation of propylene or isobutylene, attention has been attracted to a process for producing (meth)acrylonitrile by a catalytic ammoxidation of propane or isobutane in the gaseous phase, and a process for producing (meth)acrylic acid by a catalytic oxidation of propane or isobutane.
As catalysts which can be used for increasing the selectivity and yield in the reactions used in these processes, a number of oxide catalysts containing molybdenum, vanadium, niobium and antimony have been proposed.
For example, various catalyst compositions intended for producing (meth)acrylonitrile or (meth)acrylic acid with high selectivity and in high yield are disclosed in various patent documents, such as Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 9-157241 (corresponding to U.S. Pat. No. 5,750,760 and EP 767164B1), Hei 10-45664, and 2002-239382 (corresponding to U.S. Pat. No. 7,109,144 B2, US 2002/0115879 A1 and US 2006/0252954 A1).
Further, there are prior art documents which disclose the average valence of the component elements of a catalyst or disclose the atomic ratio of oxygen in a catalyst formulation. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 2002-301373 has a description about the average valence of the component elements (other than the carrier) of a catalyst. Specifically, this patent document states that the average valence is generally from 4 to less than 6, preferably from 4.5 to 5.9, more preferably from 5 to 5.8. Unexamined Japanese Patent Application Laid-Open Specification No. 2003-24790 (corresponding to U.S. Patent Application Publication No. US 2002/0183548 A1 and EP 1254708A2) states that the representative atomic ratio of oxygen in a catalyst formulation is from 3 to 4.7, relative to molybdenum.
However, the catalysts (containing molybdenum, vanadium, niobium and antimony) disclosed in these patent documents are still unsatisfactory with respect to performance and hence cannot be commercially advantageously employed.
There are known various methods for producing catalysts which can increase the selectivity for and yield of the desired product in oxidation or ammoxidation. For example, such methods for producing catalysts are disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-28862, EP 895809A1, Unexamined Japanese Patent Application Laid-Open Specification Nos. 2001-58827, 2002-301373, 2002-316052 and 2003-24790 (corresponding to U.S. Patent Application Publication No. US 2002/0183548 A1 and EP 1254708A2).
Especially, there are prior art documents which provide a teaching about a calcination method used in a method for producing a catalyst which can increase the selectivity for and yield of the desired product in oxidation or ammoxidation. For example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 9-157241 (corresponding to U.S. Pat. No. 5,750,760 and EP 767164B1) states that the calcination may be performed in an oxygen-containing atmosphere, but is preferably performed in an oxygen-free atmosphere. Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-28862 states that the calcination may be conducted using either a fluidized-bed kiln or a rotary kiln or using these kilns in combination. Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-45664 states that, prior to conducting the calcination, the catalyst precursor may be subjected to a thermal decomposition in the air to remove the most of volatile components from the catalyst precursor. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 2002-316052 states that, in the case of a continuous calcination, the calcination is performed while supplying an inert gas at a flow rate of from 500 to 10,000 N liters per 1 kg of the supplied catalyst precursor, thereby effecting a thermal decomposition of the catalyst precursor.
However, with respect to the calcination methods employed in these prior art documents, there has not yet been found an important factor greatly influencing the selectivity for and yield of the desired product which are exhibited by the obtained catalyst. Therefore, the selectivity for and yield of the desired product which are exhibited by the catalysts produced by employing the conventional methods are not satisfactory from the commercial viewpoint.
A catalyst for commercial use not only needs to exhibit a high yield at the early stage of the reaction, but also needs to maintain the yield even when the reaction is performed for a long time (specifically, 1,500 hours or more). When the yield cannot be maintained for a long reaction time, it is conceivable to take the deactivated catalyst out of the reactor and feed a fresh catalyst into the reactor; however, such replacement of the deactivated catalyst by a fresh catalyst has a problem in that the replacement operation is cumbersome, hinders the continuous operation of the reactor and is also disadvantageous from the economic viewpoint. It is also conceivable to take a measure in which the degraded catalyst is taken out of the reactor and subjected to a regeneration operation to thereby obtain a regenerated catalyst, which is then returned to the reactor; however, this measure poses a problem in that the regeneration operation takes a long time and needs a complicated regeneration equipment and/or that a satisfactory regeneration of the catalyst cannot be achieved. Accordingly, there has been a demand for an excellent catalyst which exhibits only a small lowering of the yield of the desired product in a catalytic oxidation or ammoxidation reaction. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 2002-239382 (corresponding to U.S. Pat. No. 7,109,144 B2, US 2002/0115879 A1 and US 2006/0252954 A1) discloses a catalyst which maintains the selectivity at almost the same level, although the selectivity can be maintained only for a relatively short reaction time of about 1,000 hours. However, this catalyst exhibits a low activity and hence a low conversion of propane fed; therefore, when this catalyst is used in a one pass mode of reaction, the yield of the desired product is not high. When a catalyst exhibits a low conversion of propane, it is conceivable to take a measure in which the unreacted propane is separated and recovered from the gas flowing out of the reactor and recycled to the reactor; however, this measure is disadvantageous in that the process of separation, recovery and recycling of the unreacted propane requires a large scale equipment. Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-169716 discloses a catalyst which maintains the yield at almost the same level, although the yield can be maintained only for a relatively short reaction time of about 1,300 hours. However, the working examples of this patent document employ a catalyst which contains tellurium but no antimony, and there is no specific description of a catalyst containing molybdenum, vanadium, niobium and antimony. Further, when a catalyst containing tellurium is used in a commercial scale reaction, a problem tends to arise in that the tellurium volatilizes and escapes from the catalyst with the passage of reaction time, thus destabilizing the reaction and rendering it difficult to commercially perform the reaction for a long time. In Unexamined Japanese Patent Application Laid-Open Specification No. Hei 2-2877 (corresponding to U.S. Patent No. 4,784,979 and EP 320124A), there is a description of a redox reaction of antimony and vanadium, but there is no technical concept of controlling the reduction ratio of a catalyst. In addition, it is presumed that the reduction ratio of a catalyst which is produced under the catalyst production conditions used in the working examples of this patent document would be much lower than 8%.
On the other hand, with respect to a catalyst containing molybdenum, there are cases where the catalyst is degraded by the volatilization and escaping of molybdenum from the catalyst, although the degree of the degradation is small, as compared to the degradation caused by the volatilization and escaping of tellurium. For preventing this degradation, there has conventionally been known a method in which a molybdenum compound is added to the reactor during the reaction.
For example, Unexamined Japanese Patent Application Laid-Open Specification No. 2001-213855 discloses a process for producing an unsaturated nitrile stably in a high yield by using a catalyst containing molybdenum, vanadium, niobium and antimony, wherein the process involves a step of adding to the reaction system a compensative compound comprising at least one compound selected from the group consisting of a tellurium compound and a molybdenum compound. In this patent document, the amount of the compensative compound is described to be such that the weight ratio of the compensative compound to the catalyst is equal to or less than 0.1/1, preferably equal to or less than 0.02/1. In Example 2 of this patent document, it is described that a reaction is performed for 53 hours in total, wherein during the reaction, both a tellurium compound and a molybdenum compound are simultaneously added to the reaction system, each in an amount of 0.1 g, relative to 45 g of the catalyst (namely, the weight ratio of each compound to the catalyst is 0.0022/1). This means that the amount of molybdenum compound which added to the reaction system per hour of the reaction time is such that the weight ratio of the molybdenum compound to the catalyst is as large as 0.000042/1; that is, a large amount of molybdenum compound is added to the reaction system. In the case where a molybdenum compound is added to a reaction system for the purpose of maintaining the yield, when the molybdenum compound is added in a large amount, there occur problems not only in that the cost of the molybdenum compound becomes large, which is economically disadvantageous, but also in that, when the reaction is conducted using a fluidized-bed reactor, the molybdenum compound added adheres to the heat removal coil in the reactor, thus hindering the transfer of heat to the heat removal coil and rendering it impossible to perform a stable reaction. Therefore, there has been a demand for a catalyst which has an advantage in that, in performing the conventional practice of adding a molybdenum compound to the reaction system, the amount of molybdenum compound added and the frequency of addition of molybdenum compound can be decreased to a level as low as possible.
For producing a desired product stably and economically on a commercial scale, it is especially important to maintain the yield of the desired product at a high level for more than 1,500 hours from the start of the reaction. In this respect, there has been a demand for a catalyst which can exhibit a performance such that, even more than 1,500 hours after the start of the reaction, a high yield can be maintained by the addition of only a small amount of molybdenum compound to the reaction system. However, there has not yet been known an excellent catalyst which exhibits only a small lowering of the yield of the desired product, thereby enabling the maintenance of a high yield by the addition of only a small amount of molybdenum compound during the reaction.
Further, it should be noted that, with respect to a catalyst which is commercially used in a catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase, the catalyst is required to exhibit an important performance such that, in addition to high yield and high stability of the yield with the passage of reaction time, the catalyst exhibits a moderate activity. In general, when a catalyst having low activity is used in a catalytic oxidation or ammoxidation, the catalyst is used in an increased amount for the purpose of obtaining a desired conversion of a raw material used. In such a case, however, when the activity of the catalyst is too low, disadvantages occur not only in that too large an amount of catalyst is necessary, but also in that the load on the reactor becomes large, and the size of the reactor needs to be increased.
In the case of a catalyst having too low an activity, it is naturally conceivable to take a measure in which the catalytic activity is increased by raising the reaction temperature; however, this measure poses a problem not only in that, when the reaction temperature is raised to a level which is higher than an appropriate temperature, the yield of the desired product is decreased, but also in that, in the case of an ammoxidation reaction, the ammonium used as a raw material is wastefully burnt without being used for producing the desired product. Also, the use of too high a reaction temperature is undesired because of the occurrence of adverse effects on the material of the reactor.
On the other hand, when a catalyst having too high an activity is used in a catalytic oxidation or ammoxidation, there is a problem in that the conversion of a raw material used is increased too much, leading to a lowering of the yield of the desired product and a generation of too great an amount of heat of reaction. Therefore, it is conceivable to take a measure in which the amount of catalyst used is decreased. However, this measure poses the following problem. With respect to a fluidized-bed reactor which is used for performing a catalytic oxidation or ammoxidation on a commercial scale, the fluidized-bed reactor is equipped with a heat removal coil designed for removing the heat of reaction generated during the oxidation or ammoxidation. In this case, when the amount of the catalyst used is decreased in an attempt to prevent the adverse effects of too high a catalytic activity, the decrease in the amount of the catalyst results in a decrease in the contact area between the catalyst and the heat removal coil, thus rendering it impossible to effect a satisfactory heat removal and continue the operation of the reactor. There is a further problem in that the amount of the raw material gas per unit weight of the catalyst becomes too large, which tends to cause a degradation of the catalyst. It is conceivable to take a measure in which the catalytic activity is decreased by lowering the reaction temperature; however, this measure poses a problem in that the selectivity for the desired product is decreased.
As seen from the above, there has not yet been known a catalyst which is advantageous not only in that the selectivity for and yield of the desired product in the oxidation or ammoxidation are high, but also in that the catalyst exhibits only a small lowering of the yield of the desired product even in a long reaction time, and the yield of the desired product can be easily maintained at a high level for a long reaction time, while exhibiting a moderate catalyst activity.